"We describe here an innovative strategy for understanding the so-called magnetostrophic regime of fluid flow in the Earth s core, and thus the mechanisms by which the Earth s magnetic field is sustained over time. The magnetostrophic regime is the state in which Lorentz (magnetic) forces are balanced by Coriolis (rotational) forces and pressure gradients and is thought to be the zeroth order force balance in the core. We propose a series of ground-breaking experiments using liquid sodium contained in a rapidly rotating sphere containing a differentially rotating solid inner sphere. For the first time electric current is injected into the fluid in different configurations in order that the Lorentz force is everywhere significant. Various other magnetic fields can be applied from the exterior and the interior. The influence of turbulence, viscous and magnetic boundary layers will be examined. The presence of instabilities and wave motion will be studied, and the existence of steady solutions will be naturally determined. Diagnostic measurements of magnetic fields and electrical potentials, and Doppler velocimetry will characterise the experiment. These unique experiments are backed by numerical calculations. Complementary studies will analyse the observed magnetic field over the last 400 years in the same magnetostrophic framework. An inverse method will be developed to find the initial state of the field that evolves in a manner compatible with observations. This will elucidate the interior structure of the magnetic field for the first time, determining the amplitude and morphology of the field. The importance of magnetic diffusion (Joule heating) will arise naturally, and fluid motion in the entire core will be found, allowing comparison with geodetic observations."